QUARTZ DIORITE PLUTONISM ACROSS THE OROFINO SEGMENT OF THE SALMON RIVER SUTURE ZONE, IDAHO: IMPLICATIONS FOR MAGMA GENERATION ACROSS ISLAND ARC COLLISION ZONES

In many collisional orogens, suture zones provide sharply defined boundaries between domains of distinct lithology, age, metamorphic grade, structural style, and chemical and isotopic composition among plutonic rocks. These features have been well established along the north-south striking segment of the Jura-Cretaceous Salmon River suture zone south of the 46th parallel in north-central Idaho. However, as this zone continues along the northwest-southeast striking Orofino segment to the north, lithologically similar hornblende biotite quartz diorite occurs on both sides and considerable distances (>100 km) away from the collisional boundary. Initial isotopic compositions and ages vary relatively consistently across the suture zone. Southwest of the boundary, Jurassic-Cretaceous quartz diorite intrudes Permian(?)-Triassic volcano-sedimentary strata of the Seven Devils Group across much of the Wallowa island arc terrane and yields initial Sr isotopic values <0.704. Along the suture zone, Cretaceous(?) quartz diorite intrudes the Orofino series, a volcano-sedimentary assemblage of poorly known age, and yields initial Sr isotopic values that vary from 0.704 to >0.706 across a distance of about 4 km. Northeast of the suture, Cretaceous quartz diorite intrudes Proterozoic North American sedimentary sequences of the Syringa and Belt Supergroup assemblages and yields initial Sr isotopic values >0.706. Modal compositions and major element chemistries are similar among the quartz diorite groups, but trace element chemistry differs slightly. Rare earth element chemistry shows the largest difference, with quartz diorite that intrudes continental assemblages enriched, particularly in light rare earth elements as compared with quartz diorites from the Wallowa terrane. We are testing the hypothesis that all of the quartz diorites may have been derived from similar arc basement environments, and that differences in their trace and isotopic chemistry arise from differing amounts of continental components in the source regions.